Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultra-violet (DUV) or extreme ultraviolet (EUV) light. Next, the wafer with the photoresist on top is exposed to projection light in a projection exposure apparatus. The apparatus projects a mask containing a pattern onto the photoresist so that the latter is only exposed at certain locations which are determined by the mask pattern. After the exposure the photoresist is developed to produce an image corresponding to the mask pattern. Then an etch process transfers the pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system for illuminating the mask, a mask stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example.
Ideally, the illumination system illuminates each point of the illuminated field on the mask with projection light having a well defined intensity and angular distribution. The term angular distribution describes how the total light energy of a light bundle, which converges towards a particular point on the mask, is distributed among the various directions of the rays that constitute the light bundle.
The angular distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be projected onto the photoresist. For example, relatively large sized features may involve a different angular distribution than small sized features. The most commonly used angular distributions of projection light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the intensity distribution in a system pupil surface of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the system pupil surface. Thus there is only a small range of angles present in the angular distribution of the projection light, and thus all light rays impinge obliquely with similar angles onto the mask.
Different ways are known to modify the angular distribution of the projection light in the mask plane so as to achieve the desired illumination setting. For achieving maximum flexibility in producing different angular distribution in the mask plane, it has been proposed to use mirror arrays or other spatial light modulators that illuminate the pupil surface.
In EP 1 262 836 A1 the mirror array is a micro-electromechanical system (MEMS) including more than 1000 microscopic mirrors. Each of the mirrors can be tilted about two orthogonal tilt axes. Thus radiation incident on such a mirror device can be reflected into almost any desired direction of a hemisphere. A condenser lens arranged between the mirror array and the pupil surface translates the reflection angles produced by the mirrors into locations in the pupil surface. This known illumination system makes it possible to illuminate the pupil surface with a plurality of spots, wherein each spot is associated with one particular mirror and is freely movable across the pupil surface by tilting this mirror.
Similar illumination systems are known from US 2006/0087634 A1, U.S. Pat. No. 7,061,582 B2 and WO 2005/026843 A2.
However, using a mirror array in the illumination system can also involve re-designing the illumination system to some extent. For example, the use of a mirror array typically involves an additional beam folding mechanism such as prisms or plane folding mirrors to keep the overall dimensions of the illumination system small.
In this context US 2009/0116093 A1 proposes a special prism that includes a first surface and a second surface at which impinging projection light is reflected by total internal reflection. The first surface reflects the projection light towards a surface from which the projection light leaves the prism and falls on the mirror array. The projection light reflected from the mirror array enters the prism again via this surface and impinges on the second surface. From there it is directed towards a condenser lens arranged between the prism and a pupil surface of the illumination system. Therefore the prism is similar to a conventional K prism with the exception that light is coupled out of the prism and coupled into the prism the surface through which the projection light passes twice. In a conventional K prism, the prism angle formed between the first and the second reflecting surfaces is different so that also this surface reflects all the light by total internal reflection.
Using a prism instead of mirrors for beam folding purposes can be advantageous because at present the reflective coatings of mirrors have, for the wavelengths typically used in microlithographic illumination systems, a reflectivity which is not substantially above 95%, whereas the process of total internal reflection results in a reflectivity of nearly 100%.
However, in the pupil surface of the illumination system disclosed in the afore-mentioned US 2009/0116093 A1 the light intensity distribution is often not satisfactory. In particular, there are undesired light contributions to the intensity distribution in the pupil surface. These light contributions can perturb the angular light distribution of projection light illuminating the mask.